re: barrel bore sizes and different size ammo?

All Firearms come in different calibers or gauge's.* Each caliber has its own purpose depending on the size or type of Game you want to Hunt.* Some are made for Big Game, and some are made for Small Game.*

The US uses the inch system, hence .30 caliber is .30 inch+/-.

Most of the rest of the world uses the metric system, hence 9mm.

Some variations, like 30-06 and 25-06 arise because the cartridge is a modification from a similar cartridge or because it indicates the powder load like the 45/70 (45 caliber and 70 grains of black powder), The first number indicates the caliber of the bullet.

In SOME cases they are so near identical as to make no difference, diameter wise, of the bullet although the cartridges varied.

5.56mm is .223 caliber, 7.62mm is .30 caliber, 9mm is .38 caliber, etc.

Shotguns, with the exception of the .410, are based on the weight of lead balls, just like "old" cannon were.

A 12ga size is 12 balls to the pound, and the old 5 pounder (cannon) used a ball that would weigh 5 pounds if a solid ball was used. (In practice most balls were hollow and explosive filled!)

It is confusing due to the lack of industry standard naming conventions. There are industry standards that are agreed upon with regard to specifications, through an intternational organization called SAAMI, it's just the naming of the different calibers that are allowed some flexibility.

Also remember, you are in many cases dealing with calibers older than 100 years and ones developed in different countries, languages, using different standards of measurement and by companies or developers that chose a caliber name based on marketing preferences.

.30 caliber for example. Many of the countries involved in WWII used a ".30 caliber" main battle rifle, but that is only a general range of diameter. The Brits had .303, the US .30-06, the Russians 7.62x54r. The Germans used a bit bigger at 8mm (7.92, or actually .32 caliber).

9mm is another that is very confusing. 9x19mm, 9mm Lugar, 9mm Parabellum and 9mm NATO are the same ammo. Then there's the Soviet 9x18, aka Makarov, plus 9mm Largo (Spanish), 9mm Kurz (.380 or 9mm Browning Short), all different.

Also, you must consider that the word "caliber" has come to describe the cartridge specification, not just the diameter of the bore or projectile. Different "calibers" of cartridges can actually use the exact same projectile diameter, it's just that cartridge length and shape may differ greatly. There is a lot more to consider than just diameter.

I like to joke that the extremely complex naming variations help the experts weed out the novices.

Don't forget that marketing and business play a big part in this as well. The legendary arms making giants of a century ago put their name on calibers they developed (Winchester, Colt, Remington, Mauser, etc) as part of their name recognition, which today has become legend. Also, many independent designers like John Moses Browning put his name on many calibers and guns, although he was a free-lancer (Browning Arms was founded after he died!).

This is why you see so many "new" calibers being introduced today, many for no good reason, as the new arms giant companies (Glock, SIG, etc.) want to get their name on a caliber too.

The 30-06, 30-30, and .308 are all 30 caliber rounds. The first was a .30 cal designed in 1906. Different gun companies make different cartridges, and they name them what they want to.

The mm designations are for rounds made by companies outside the USA, or that have been made for, or adopted by, the military, so they are using metric measurements.

Did you know the 12 gauge is named this because if you take 12 100% round lead balls that are the same diameter as the barrel, they will add up to exactly one pound? Same for 10, 16, 20, and 28 gauge. However, the 410 shotgun is measured like a rifle. That is, it's .41 caliber (.41 inches in diameter).

well, here is a simple form,, 1/4 inch is .25 caliber,, 1/2 inch is .50 caliber so then in mm just some basic math.ok tu eddie

See the link, below.